Added a new plugin which adds completion for ripgrep (rg)

Based on the existing "cargo" plugin.

plugins/ripgrep/README.md

+# ripgrep
+
+This plugin adds completion for the text search tool [`ripgrep`](https://github.com/BurntSushi/ripgrep), also known as `rg`.
+
+To use it, add `ripgrep` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... ripgrep)
+```
+
+Completion is taken from the ripgrep release [`0.10.0`](https://github.com/BurntSushi/ripgrep/releases/tag/0.10.0).
+
+Updated on September 27th, 2018.